Regardless of the technologies you use to design and implement a CCTV security system, there are a number of issues that must be addressed in all situations. First, what information do you want the system or component to provide? There are three possible answers:
• Detection – indicate something is happening in the field of interest
• Recognition – determine exactly what is happening
• Identification – determine who is involved in the activity
But overall, all cameras are composed of three basic elements
- Image sensor – converts light image to electronic signals
- Lens – gathers light reflected from a subject
- Image processing circuitry – organizes, optimizes, and transmits signals
CCTV cameras are available in monochrome, color, and day/night (combines color and monochrome).
Monochrome’s advantages are higher resolution, less light required, and generally lower cost. Color, on the other hand, offers better overall representation of a scene (with proper light), as well as improved capabilities for identification and prosecution. Day/night cameras offer the best of both worlds – and they are increasingly becoming the camera technology of choice for both outdoor and indoor applications.
Image Sensors
A charge-coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value.
The heart of the modern CCTV camera is the Charge Coupled Device (CCD) sensor. A CCD consists of a flat array of tiny, light-sensitive photodiodes that converts light into an electrical signal. Each diode produces a voltage that’s directly proportional to the amount of light falling on it. No light would produce zero voltage, and therefore, a black level. Maximum light would produce a maximum voltage (a white level). In between these extremes are shades of grey. In the case of a color camera, a chrominance signal is superimposed onto the luminance signal to carry the color information.
Video Signal
All video motion images are actually made up of still images — or frames. Each frame is composed of two fields. One field of video is created when the CCD is scanned across and down exactly 262 1/2 times – and this is reproduced on your monitor. A second scan of 262 1/2 lines is exactly one-half of a line down and interlaced with the first scan to form a picture with 525 lines. When these two fields are properly synchronized and interlaced in a 2:1 ratio, they form a complete still frame of video. CCTV cameras use AC voltage to synchronize this process of creating motion video.
In countries like the US that use 60 Hz (cycles) alternating current, each second of video contains 60 fields, which forms 30 frames. In Europe and other regions using 50 cycles, there are 50 fields and 25 frames of video per second. To the human eye, these frames of video appear as moving images.
